1. Field of the Invention
The present invention relates to an image forming apparatus, e.g., a digital copier or a facsimile device. The image forming apparatus includes a feeding device having a transferring and release mechanism for a driving force. The feeding device is located upstream of registry rolls and located downstream of a sheet supplying and separating part in the sheet conveyance direction.
2. Discussion of the Background
In image forming apparatuses (e.g., electrostatic copying machines, printers, and facsimiles) that expand image data to form images based on a difference of the expanding period in each image data, a sheet conveyed to registry rolls remains there for a time corresponding to the expanding period. Then the sheet is conveyed synchronously with the writing action. While a sheet stops at the registry rolls, a sheet feeding device located downstream of a sheet supplying and separating part must stop feeding the next sheet.
In a conventional image forming apparatus, an electromagnetic clutch located between the registry rolls and the sheet supplying/separating part is used to transfer and release a driving force to the feeding device. When the clutch transfers the driving force to the feeding device, a coil equipped in the clutch is electrically energized. When the clutch releases the driving force, no electrical power is applied to the coil.
In recent years, as demand for higher speed printers and digital copiers has increased, sheet feeding speed has become higher and the space between sheets has narrowed. As the space between sheets has narrowed, the stopping period of the sheet feeding device has been reduced to the above-mentioned expanding time for the image data.
The expanding time is relatively short (about 1.5 seconds) except in peculiar cases. Accordingly, the clutch of the sheet feeding means becomes active for long durations. The coil, as a result becomes hotter which causes a static frictional torque of the electromagnetic clutch to decrease. As a result, the electromagnetic clutch can become faulty which causes miss-feeding of the sheet and the sheet is more easily jammed.
Further, when an image forming apparatus has plural sheet feeding devices and the distance between the sheet feeding devices and the registry is long, in order to resolve a jam condition with a single lever, driving loops coupled to each sheet feeding device are required to move all the sheet feeding devices.
Thus, the electromagnetic clutch is required to have a capacity that can transfer a driving force from the lever to the all the driving loops. When a load torque of the electromagnetic clutch from the lever is high or when the sheet supplying/separating device is united with the sheet feeding device, plural clutches and other driving loops specially designed to resolve sheet feeding jams are required. The addition of these components increases the cost of the apparatus.